A conventional inverter circuit having a semiconductor switching element and a smoothing capacitor is equipped with a substrate whereon a conductor pattern is formed and the semiconductor switching element is mounted, a case carrying the substrate and forming a shell, a molded substrate carrying the smoothing capacitor, whereon a wiring pattern made of a conductive metal is formed by insert molding with a mold resin, and wherein the smoothing capacitor is connected to the wiring pattern, and a molded substrate terminal having a tap placed on the case. It is suggested a power module wherein an inverter circuit is configured by fixing the molded substrate to the case by securing the molded substrate to the tap of the molded substrate terminal with a screw to provide electrical connection therebetween (For example, refer to Patent literature 1).
Further, it is suggested an inverter circuit configured by mounting and wiring an electrolytic capacitor for power smoothing on a power wiring board, which is more expensive than a printed circuit board, and by fastening power elements with a screw (For example, refer to Patent literature 2).
Patent literature 1: Japanese Patent No. 3649133
Patent literature 2: Unexamined Patent Publication No. 63-110960
In the conventional circuit described in Patent literature 1, a shell case for carrying a board whereon a switching element is mounted is required. It is necessary to form electrodes and to perform insert molding to form electrodes on this shell case. Therefore, there is a problem that materials and additionally a die, a molding machine, processing for forming electrodes and performing insert molding are necessary. Further, there is another problem that when a circuit configuration is changed, wherein change of the die is required, and it is impossible to handle a model change in a small lot.
As for a substrate, a metal substrate is used which is a metal whereon an insulator is shaped, over which a copper foil is applied, having both a radiation function and a power wiring function. This is a method of mounting a radiation surface of semiconductor components on a substrate side, wherein the cost per unit area of a base material of the substrate itself is 10 to 20 times as expensive as that of a printed circuit board. Therefore, even if the area of the substrate can be reduced, this does not lead to a cost reduction commensurate with that in a case of using a printed circuit board.
The substrate is significantly expensive in the cost per square meter in comparison with a substrate whose function is restricted to a wiring function, and is not economical. Additionally, to mount a semiconductor on the substrate, there is a need to perform wiring of a gate terminal of a switching element, which is originally operative with a low voltage and a low electrical current and which does not need radiation, on an expensive metal substrate, to makes it more uneconomical. Further, since wiring of a copper foil is performed by etching, thickness of a copper foil results in a long etching time as in the printed circuit board. Furthermore, radiation of a radiation component, such as a power IC, is performed by dissipating the heat of a radiation surface of the radiation component through a copper foil and an insulator, and further through a metal plate toward a radiating fin. Such a configuration is wasteful and uneconomical rather than in fixing the radiating fin directly to a component.
Further, since a smoothing capacitor large in size is mounted on an insert-molded substrate which requires a die and a molding machine, it is necessary to cover the cost of the die and the molding machine by mass production for cost reduction. To increase the number of the insert-molded substrate to mount the smoothing capacitor, there is a need to use the same shape of substrates for products with a plural range of capacities. In this case, it is necessary to standardize the smoothing capacitors with that of a largest capacity model. Therefore, by contrast, unnecessary materials are used for small capacity models large in amount. Since the smoothing capacitor is one of the largest components in the inverter circuit along with the reactor, the unnecessary materials are significant compared to in a case of mounting on a printed circuit board.
Further, since the case forming a shell carrying the substrate whereon the switching element is mounted has a structure wherein the electrodes have a planar contact with the substrate, and the electrodes and the substrate are joined with soldering, the solder strength against heat shrink is small.
Further, a conventional circuit lacks area efficiency since a power device is directly mounted on a printed circuit board, and a high voltage and a high electrical current wiring, which requires a pattern width and a pattern-to-pattern spacing, is formed on the printed circuit board as a planar wiring. Therefore, the electrical component case itself is large in size.
Furthermore, there is a large wiring inductance and a large developmental noise. Moreover, temperature rise at a foot part of the power module is large so that there is a design constraint on solder life.
Further, since etching is used for forming a copper foil wiring on the printed circuit board or the metal substrate, it takes time for etching when forming a wiring for a power wiring with a thick copper foil, which increases unit cost of the board. When the wiring is formed on the same board, a thick copper foil is used even for a fine wiring such as a control wiring, and unnecessary materials are used.
Furthermore, in the conventional inverter circuit described in Patent literature 2, since the inverter circuit is configured by fixing power elements with screws, extra screws are needed and the cost is increased. Since it takes more time and cost to process screw tightening, it does not suit for mass production of a circuit, such as an inverter for an air conditioner to be produced in large quantities. Meanwhile, it may be also considered a method to fix a connector or a metal terminal, etc. by soldering to join the connector or the metal terminal, etc. with a printed circuit board. However, another connector or wire lead becomes necessary, and extra time for forming lead joints is needed. Further, since screw tightening and connector connecting are of human works, there is a high provability that defective joints are generated. If defective joints are generated, there is a high risk of temperature rise, a smoking and an ignition due to a bad connection at a power unit. To avoid these, it is necessary to take time for inspection process.
Further, in the conventional inverter circuit described in Patent literature 2, the electrolytic capacitor for power smoothing is mounted and wired on a power wiring board, which is more expensive than a printed circuit board. However, it is not economical to mount on a power wiring board an electrolytic capacitor, whose wiring is originally simple for the large area of the component. Further, the capacitance of the electrolytic capacitor varies greatly depending on the models. When a substrate is standardized according to one with the largest capacitance value, a model having a large volume of sales and using an electrolytic capacitance with a low capacitance value produces much waste.
The present invention is made to solve the problems as shown above, and one of the purposes is, while maintaining a low-noise and low-loss power wiring due to lowering inductance, to obtain a motor drive circuit and an outdoor unit for an air conditioner using the same, which can flexibly support change of a model at a low price and in a small lot without using unnecessary materials, wherein a stress in a soldering part due to self-heating is low, a solder reliability is high, and design constraints are small.